*{
    margin: 0;padding: 0;
}
#main{
    margin: auto;
    position: absolute;
    top: 0;left: 0;bottom: 0;right: 0;
    width: 400px;
    height: 200px;
    background: #ccc;
}
#leftTopArea{
    width: 50px;
    height: 50px;
    background: #fc0;
    border-radius: 0 0 100% 0;
    position: absolute;
    top: 0;
    left: 0;
}
#rightBottomArea{
    width: 50px;
    height: 50px;
    background: #fc0;
    border-radius: 100% 0 0 0;
    position: absolute;
    bottom: 0;
    right: 0;
}